2.3. Experimental section 1. Plant material
2.3.6. Biological assays
The experimental procedures for cytotoxic and anti-HIV assays were previously reported (Fobofou et al., 2015b).
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Chapter 3
Tricyclic acylphloroglucinols from Hypericum lanceolatum and regioselective synthesis of selancins A and B
Graphical abstract*
Highlights
• 11 new acylphloroglucinol derivatives were isolated from the leaves of H. lanceolatum
• Two unprecedented skeletons
• The known 3-O-geranylemodin is reported from the genus Hypericum for the first time
• The new compounds selancins A and B were synthesized regioselectively
• 1H NMR profiles guided fractionation for new compounds discovery
*This chapter (with slight modifications) was published: Fobofou, S.A.T., Franke, K., Porzel, A., Brandt, W., Wessjohann, L.A., 2016. J. Nat. Prod. 79, 743-753. Reprinted (adapted) with permission from the American Chemical Society (ACS). Copyright (2016) American Chemical Society.
absolute configuration
50 Abstract
The chemical investigation of the chloroform extract of Hypericum lanceolatum Lam. guided by 1H NMR, ESI-MS, and TLC profiles led to the isolation of 11 new tricyclic acylphloroglucinol derivatives, named selancins A-I (3.1-3.9) and hyperselancins A-B (3.10-3.11), along with the known compound 3-O-geranylemodin (3.12) which is described for a Hypericum species for the first time. Compounds 3.8 and 3.9 are the first examples of natural products with a 6-acyl-2,2-dimethylchroman-4-one core fused with a dimethylpyran unit. The new compounds 3.1-3.9 are rare acylphloroglucinol derivatives with two fused dimethylpyran units. Compounds 3.10-3.11 are derivatives of polycyclic polyprenylated acylphloroglucinols related to hyperforin, the active component of St. John’s wort. Their structures were elucidated by UV, IR, extensive 1D and 2D NMR experiments, HR-ESI-MS, and comparison with the literature data. The absolute configurations of 3.5, 3.8, 3.10, and 3.11 were determined by comparing experimental and calculated electronic circular dichroism (ECD) spectra. Compounds 3.1 and 3.2 were synthetized regioselectively in two steps. The cytotoxicity of the crude extract (88% growth inhibition at 50 µg/mL) and of compounds 3.1-3.6, 3.8, 3.9, and 3.12 (no significant growth inhibition up to a concentration of 10 mM) against colon (HT-29) and prostate (PC-3) cancer cell lines was determined. No anthelmintic or antiviral (HIV) activity was observed for the crude extract.
Keywords: Hypericaceae, Hyperselancins, prenylated and polycyclic compounds, natural products, chiroptical studies, 1H NMR guided isolation.
51 3.1. Introduction
The great scientific interest and economic value of Hypericum perforatum (St. John’s Wort), a medicinal herb mostly used for the treatment of mild to moderate depression, entailed the investigation of bioactive metabolites from other Hypericum species (Porzel et al., 2014; Fobofou et al., 2014).The most common secondary metabolites found within the genus Hypericum include phloroglucinols, xanthones, naphthodianthrones, flavonoids, and coumarins (Farag and Wessjohann, 2012; Athanasas et al., 2004; Crockett et al., 2008; Shui et al., 2012; Tanaka et al., 2009; Hecka et al., 2009; Kusari et al., 2008, Ang’edu et al., 1999). Acylphloroglucinols, of which hyperforin from H. perforatum is an outstanding example, are among the most relevant bioactive compounds isolated from Hypericum (Uwamorin and Nakada, 2013; Liu et al., 2013; Grey et al., 2007). Their structures and biological activities have attracted much attention in the medicinal and synthesis chemistry fields since the isolation of hyperforin in 1975 (Liu et al., 2013). They possess, among others, cytotoxic, antidepressant, antibacterial, and anti-inflammatory activities (Crockett et al., 2008; Shui et al., 2012; Liu et al., 2013; Verotta, 2003). The phloroglucinol core (1,3,5-trihydroxybenzene) of these compounds is often substituted by prenyl or geranyl moieties that are susceptible to cyclization and oxidation processes affording bi- or tricyclic derivatives as well as complex cage compounds (Athanasas et al., 2004; Beerhues, 2006; Liu et al., 2013).
Recently, a revised structure for adhyperfirin with a C-methylated phloroglucinol core, named hyperpolyphyllirin from H. polyphyllum, was reported (Porzel et al., 2014).
Hypericum lanceolatum Lam. (Hypericaceae) has been prioritized for chemical investigation as part of a metabolomics driven isolation project aiming to study and compare the metabolomes, biosynthesis, and biological activities of prenylated aromatics and to find new or potential lead compounds from plants including various Hypericum species and accessions (Fobofou et al., 2014;
Porzel et al., 2014; Farag and Wessjohann, 2012; Heinke et al., 2012; Heinke et al., 2013).
H. lanceolatum is a medicinal plant occurring in the mountainous region of West Cameroon (Central Africa). Previous studies on this plant revealed the presence of xanthones, a polyprenylated phloroglucinol (isogarcinol), and terpenoids (Wabo et al., 2012). In Cameroonian traditional medicine, Hypericum plants are multipurpose remedies commonly used for the treatment of tumors, skin infections, epilepsies, infertility, venereal diseases, gastrointestinal disorders, intestinal worms, and viral diseases (Fobofou et al., 2014; Wabo et al., 2012; Zofou et al., 2011). Recently, the first isolation and structural elucidation of biscoumarins from Cameroonian H. riparium were reported (Fobofou et al., 2014).The aim of the present study was
52
to isolate and characterize the chemical constituents of H. lanceolatum and evaluate their biological activities.